In large saucepan over medium heat, cook onions in margarine until tender. Stir in flour, salt, sage and pepper; cook until bubbly. Stir in broth, milk and mushrooms. Cook and stir until mixture boils and is slightly thickened. Stir in remaining ingredients except Swiss cheese. Spoon into ungreased 12x8-inch (2-quart) baking dish; sprinkle with Swiss cheese.

Bake, uncovered, in preheated 350-degree oven 20 to 30 minutes until hot and bubbly. If desired, sprinkle with additional chopped fresh parsley.